This study was undertaken to investigate the interaction of immunoglobulins with various cell surface receptors and immune amplification molecules. Covalent complexes have been prepared by specifically cross-linking antibody molecules through antigen interactions at their combining sites and nonspecifically by chemically cross-linking lysines of neighboring antibody molecules. Oligomers of defined size have been isolated. We have been able to define the affinities and kinetics of binding of IgG and IgG oligomers to the Fc receptors of various cell types. In addition, subclass specificity for murine IgG myeloma proteins has been determined. Preliminary studies of the binding properties of isolated populations of human cells have been carried out. The binding properties of oligomers have been correlated with their ability to block antibody dependent cell mediated cytolysis using three populations of effector cells. Cross-linked proteins of the IgE class have been used to show that the allergic triggering unit for mast cells consists of two cross-linked IgE molecules. The fixation of complement by monomeric IgG has been characterized and studies on the interaction of the complement system with IgG oligomers have been initiated. BIBLIOGRAPHIC REFERENCES: Segal, D.M. and Hurwitz, E.: Binding of affinity cross-linked oligomers of IgG to cells bearing Fc receptors. J. Immunol. 118:1338, 1977. Hurwitz, E., Zatz, M.M., and Segal, D.M.: The biological activity of affinity cross-linked oligomers of rabbit immunoglobulin G as tested by antibody dependent cell-mediated cytolysis (ADCC). J. Immunol. 118: 1348, 1977.